Integrated Circuit (IC) chips consist of thousands of transistors. These transistors are organized into circuits for performing certain logic functions. Data and power is communicated to the IC via leads. Typically these leads consist of two parts, namely, an inner lead, which is a thin wire connected to bonding pads on the integrated circuit, and an outer lead, which is thicker and more sturdy than the inner lead.
The IC and the inner lead are usually entirely encapsulated in some form of shell, e.g., epoxy. The connectability of the IC to external circuitry is accomplished by way of the outer leads. The outer leads are given some shape to facilitate the connections between the IC and the external circuitry, e.g., bent so as to enable the positioning of the module in a socket, or bent to provide a suitable surface for soldering to a printer circuit board.
During the manufacturing of ICs the leads are attached to the IC. The first step is the repetitive manufacture of minature printed circuits on a metal tape. Each miniature circuit on the tape is bonded to an IC. The bonds are made between the inner leads and gold bonding pads on the perifery of the IC. These bonds are created simultaneously as the tape passes over an IC attachment tool. At the end of this step, the tape will consist of inner lead circuits with IC bonded to each such circuit.
A lead frame is the metal frame which holds the leads of a circuit package in place before encapsulation. Photolithography, e.g., etching, is used to manufacture lead frames for the semiconductor industry. Manufacturing of lead frames includes both stamping and etching techniques. Etching is used to create finer geometries, i.e., designs in which the individual leads are relatively thin. The material used for manufacturing lead frames may be either in the form of sheets or tape reels.
When the material for the lead frames is in the form of tape reels, the final step of the lead frame manufacturing process, the etching, is done using reel-to-reel equipment, wherein the material is transferred from a product pay-off reel to a product take-up reel. The etching process occurrs between these two reels.
To expand the manufacturing capacity, certain manufacturers of lead frames have adopted multi-rail systems. In such systems one common rail containing multiple tracks of lead frames is passed through the etching process, thereby manufacturing multiple lead frames in parallel. These multiple tracks are then separated into individual rails, each having a width containing the leads that correspond to one IC.
A copper shearing process is one method of separating the common rail into multiple rails. A disadvantage of this method of separating the rails is that the shearing process can create product distortion. Furthermore, the copper shearing process requires an extra handling step, which occassionally leads to damaged products.
"Etch slit" is an alternative method of separating the common rail into multiple rails. Etch slit is a simple modification to the photomask used in the lead frame manufacturing process to create a space between the product rails. This modification to the photomask allows the etch solution to penetrate and "slit" the product into multiple rails.
A problem due to the etch slit step is that as the common rail is separated into multiple rails, the tension in the individual rails varies. If the individual rails are taken up on a common spool then, there is no control of the tension in the individual rails. Thus, there may be slack in some rails and too much tension in others.
A disadvantage of not being able to control the tension in the is that it is more difficult to control the precision with which the lead frames are etched. In some cases the lead frames must be within a width tolerance of .+-.1 mil. With excessive slack and or tension it is difficult to meet those tolerance requirements.
Clutch drive systems provide one crude solution to the tension control problem in multi-rail systems. In a clutch drive system one reel is driven by a direct drive system. The master reel, in conjunction with the direct drive which rotates it, is used for closed-loop feedback control, i.e., a sensing device determines the rate at which the direct driven reel should turn, and the direct drive rotates the reel at that rate.
The other reels, i.e., the ones not driven by the direct drive, are powered by friction. Friction plates, or spacers, are located between the direct driven reel and the adjacent friction driven reels. If more than one friction driven reel is used, then friction plates may also be located between the various friction driven reels. The friction between the reels enables the various reels to turn in parallel, and allows the friction driven reels to turn at a rate which is slower than the rate of rotation of the direct driven reel.
However, in clutch drive systems the adjacent friction driven reels are not able to turn faster than the direct driven reel. Due to differences in reel diameter at times it is required for the adjacent reels to turn at a higher rate than the direct driven reel. Because it is impossible in a clutch drive system to have the friction driven reels turn faster than the direct driven reel, such systems require human intervention to take up any slack. Furthermore, clutch drive systems rely on friction to control the relative rate of rotation of the various take-up reels in the system. The friction in the clutch drive systems is caused by friction plates rubbing against each other or against reels. This rubbing of material causes the production of particules. Particle production is undesirable in the production of semiconductor integrated circuit modules.
It is therefore desirable to provide a system and method for enabling multiple take-up reels to rotate at different rates, and overcomes the problems described above with reference to the prior art.